early 944 brakes, stock MC....dosent feel right

[super vw]

Well i have been running my early N/A 944 setup (from and rear stock) with the stock M/C (19 front 23 rear) and have to say thay it is front bias (to much really, as i lock up to early due to the very little weight transfer of a 1303 under breaking). i have done 6 Autocross sessions with this set up, and have found adding weight (60 lbs) to the front of the car HELPs A LOT with breaking AND turning (no more scrunbbing in mid turn). anyways im going to swap the front to rear lines Ex: 23mm front and 19mm rear. this should give me more of the rear bias that im looking for. If i get time this week to do it i will post back. Im going to do another Autocross session this weekend.

[1303R]

i am right in thinking the 23mm front circuit is the end nearest the driver, 19mm rear is furthest?

[speedy]

i belive this is true, in relation to this thread i found this on a 944/924 tech website (sorry 1303r cannot be arsed to type it all

The final note that is relevant here is on brake biasing or proportioning. The front wheels must develop more brake torque (braking power, if you will) than the rears, due to weight transfer under braking, in order to prevent the rear wheels from locking first and spinning the car. This can be achieved many different ways; in modern cars it is now being handled electronically. However, in the 924's, 931's, and 944's, it's handled by hydraulic component sizing (to get the correct ratio front-to-rear). In the 924 and 931, the master cylinder has the same bore size for each circuit, and the front and rear calipers use different sizes to achieve the correct proportioning. The 944 uses a master cylinder with different bore sizes front and rear - since the fronts and rears are on distinct circuits. Therefore, for a given amount of pressure on the brake pedal, a 924/931 will develop identical pressure on all four corners, whereas the 944 will develop more pressure up front than at the rear. It then becomes clear why the brake corner components must be matched with the master cylinder when doing a brake conversion -
The most important thing to remember from this - use the correct brake master cylinder from the car from which the calipers came from when converting, and ensure that the brake lines are plumbed correctly.


this seems to back up my theory on larger overall volume of liquid displaced but lets not forget the weight distribution of a 944 was between 45/55 and later 50/50 ,although not 100 percent certain i think a bugs weight dist will be atleast 40/60 35/65 front to back therefore requiring more rear bias than a 944
any thoughts on this are welcome
cheers jon

[1303R]

ok but that still doesn't tell you anything.also where did it come from? grabbing stuff from random forums or the like is not real infomation. i'd like to here from the people who make brake systems. knowing what works for some people doesn't really tell me anything, i'd much rather find out why and how it works. sorry if this sounds off, i'm here to learn, just wanna make sure i'm learning the right info.

[speedy]

i agree, there is alot of BS**t written on forums that is why when i give an opinion or infomation that i am not 100% sure about i explain where it came from it is then up to the person recieving the info what they do with it anyway the above came fromhttp://www.924.org/techsection/9brak...%20description when investigating the stepped m/c i have come to realize that there are many factors involved in brake system design it is mind bending , one thing is for sure standard 944 biasing is not the ideal for a beetle as it was calculated for a front engined car(more weight over the front wheels) , on the road under normal conditions this is not an issue but on the track or under heavy use braking distances will be larger than for a car with a well setup bias because the rear brakes will not be working as hard as they could .In theory a bigger set of brakes with bad biasing will increase braking distance over a well biased smaller set of brakes . i think the thing that brought home the importance of the many considerations when designing a system is the fact that on a perfectly balanced braking system(front locks just before back) fitting softer compound tyres on the front alters the biasing because the tyre will then give more grip meaning the rears could lock up first hence the need for a biasing valve to correct this.
anyway enough ramblings
here is a list of some of the considerations when designing an optimal braking system
front to rear weight distribution
spring rate on suspension (effects weight transfer under braking )
ratio of tyre size front to back
weather the car is lowered
downown force of fitted spoilers etc
tyre compound
disk size
area of the pads and compound of the pads used
ratio of calipers and piston size etc
there are others but i cannot remember them
After studying a picture of m/c i now realize that Wally was right ,the Fand R m/c pistons can and must work independantly of each other , after you realize this then it is simple to see how the mechanical advantage formulea will hold true . If you apply 100kg of force to the front piston this will apply 100kg of force to the front caliper and then 100 kg to the rear piston , because the volume of liquid beetween the m/c pistons remains constant the front pistons resting position is dependent on the rear pistons position when it is at 100kg pressure. The system will balance out when both front and back pistons are at 100 kg at this point the mech advantage ratios hold true , sorry it has taken me this long to understand it (must be slow)
Hope this helps
jon

[speedy]

OK here is the thing if the above is true and brake pressure ratio is caliper to m/c piston ratio , you have to take into account the fact that the front 23mm piston is pushing a 19mm back piston hydralicaly in the master cylinder .In the calculation this gives a ratio of 1.9 on area guess what the equivelent areas are now 23.8 is 444 and the 19.05 is 443 so if the 23 is nearest the driver it will supply more fluide but not effect the bias at all ie harder pedel on 944 na calipers , maybe this is the reason some people swear 23 on the rear is better and some 23 on the front is better , it seems strange that porsche picked these two bore sizes that when area is calculated as a ratio of each other give the same surface area , could anybody chime in that has infact run the m/c both ways , i would be interested to hear your results and if they match with my calculations .I rang a brake specialist(allegedly) and due to the fact split bores are not that common he could not give me a definative answer , infact no answer
cheers jon

[Jeza]

Have a read of the 914 tech articles on the Pelican pages, that might help explain a few things.

This one in particular may answer some of your master cylinder questions

http://www.pelicanparts.com/techarti...rake_calc2.htm

I have 944 NA brakes front and rear on a 1302 and I find them okay on the street and once warm on the track I really like the way they feel and brake. They may not be perfect, but I suspect they offer better front to rear ratio than most people think. I use the 23.8mm side to the fronts as designed by Porsche. If you read the above tech article you'll see why it is done this way.

Friends with quicker cars than me would like to upgrade their 944 NA's (also on a 1302), but I really think it comes down to what you want and the use you give your car.

Out of curiousity have you compared the front to rear hydrallic ratio of this set up to early 911's, which are more tail heavy than our Beetles are.

Cheers
J